The present invention relates to a system using a work string for performing a downhole operation in a well and more particularly includes a bottom hole assembly disposed on a composite umbilical made up of a tube having a portion thereof which is preferably non-metallic. In using the well system for drilling the well, the bottom hole assembly includes a power section for rotating a bit and a propulsion system for moving the bottom hole assembly within the well.
Many existing wells include hydrocarbon pay zones which were bypassed during drilling and completion because such bypassed zones were not economical to complete and produce. Offshore drilling rigs cost approximately $40 million to build and may cost as much as $250,000 a day to lease. Such costs preclude the use of such expensive rigs to drill and complete these bypassed hydrocarbon pay zones. Presently, there is no cost effective methods of producing many bypassed zones. Thus, often only the larger oil and gas producing zones are completed and produced because those wells are sufficiently productive to justify the cost of drilling and completion using offshore rigs.
Many major oil and gas fields are now paying out and there is a need for a cost effective method of producing these previously bypassed hydrocarbon pay zones. The locations and size of these bypassed hydrocarbon zones are generally known, particularly in the more mature producing fields.
To economically drill and complete the bypassed pay zones in existing wells, it is necessary to eliminate the use of conventional rigs and conventional drilling equipment. One method of producing wells without rigs is the use of metal coiled tubing with a bottom hole assembly. See for example U.S. Pat. Nos. 5,215,151; 5,394,951 and 5,713,422, all incorporated herein by reference. The bottom hole assembly typically includes a downhole motor providing the power to rotate a bit for drilling the borehole. The bottom hole assembly operates only in the sliding mode since the metal coiled tubing is not rotated at the surface like that of steel drill pipe which is rotated by a rotary table on the rig. The bottom hole assembly may include a tractor which propels the bottom hole assembly down the borehole. One such tractor is a thruster that pushes off the lower terminal end of the coiled tubing and does not rely upon contacting or gripping the inside wall of the borehole. The depth that can be drilled by such a bottom hole assembly is limited.
One such self-propelled tractor is manufactured by Western Well Tool for propelling a near conventional bottom hole assembly in the borehole. The propulsion system includes an upper and lower housing with a packerfoot mounted on each end. Each housing has a hydraulic cylinder and ram for moving the propulsion system within the borehole. The propulsion system operates by the lower packerfoot expanding into engagement with the wall of the borehole with the ram in the lower housing extending in the cylinder to force the bit downhole. Simultaneously, the upper packfoot contracts and moves to the other end of the upper housing. Once the ram in the lower housing completes its stroke, then the hydraulic ram in the upper housing is actuated to propel the bit and motor further downhole as the lower packerfoot contracts and resets at the other end of the lower housing. This cycle is repeated to continuously move the bottom hole assembly within the borehole. The tractor can propel the bottom hole assembly in either direction in the borehole. Flow passages are provided between the packerfeet and housings to allow the passage of drilling fluids through the propulsion system.
Various companies manufacture self-propelled tractors for propelling the bit and pulling steel coiled tubing in the well. These tractors include self-propelled wheels that frictionally engage the wall of the borehole. However, there is very little clearance between the wheels of the propulsion system and the wall of the borehole and problems arise when the wheels encounter ridges or other variances in the dimensions of the wall of the borehole. Further, at times there is an inadequate frictional engagement between the wheels and the wall of the borehole to adequately propel the tractor.
Other companies also offer tractors to walk the end of a wireline down a cased borehole. However, these tractors engage the interior wall of a casing having a known inside dimension. One such tractor is manufactured by Schlumberger.
The use of metal coiled tubing has various deficiencies. Metal coiled tubing tends to buckle the deeper the bottom hole assembly penetrates the borehole. Buckling is particularly acute in deviated wells where gravity does not assist in pulling the tubing downhole. As the tubing buckles, the torque and drag created by the contact with the borehole becomes more difficult to overcome and often makes it impractical or impossible to use coiled tubing to reach distant bypassed hydrocarbon zones. Further, steel coiled tubing often fatigues from cyclic bending early in the drilling process and must be replaced. It has also been found that coiled tubing may be as expensive to use as a conventional drilling system using jointed steel pipe and a rig.
The bottom hole assembly may also include an orienting tool such as a bent sub or housing for directing the trajectory of the borehole. Some types of orienting tools may be adjusted from the surface. Often, prior art orienting tools require a 360.degree. rotation to ratchet to a new direction of inclination.
The bottom hole assembly may include various sensors such as a gamma ray and inclinometer instrument package adjacent the bit and a multiple depth dual frequency borehole compensated resistivity tool. These tools produce data indicating the inclination and azimuth of the bit and the position of the bottom hole assembly with respect to the formation. The bottom hole assembly may also include other sensors for providing other data relating to the borehole, such as gyroscopic survey data, resistivity measurements, downhole temperatures, downhole pressures, flow rates, velocity of the power section, gamma ray measurements, fluid identification, formation samples, and pressure, shock, vibration, weight on bit, torque at bit, and other sensor data.
Prior art bottom hole assemblies for rotary drilling and for use with metal coiled tubing include electronic components for collecting data, processing the data downhole, and transmitting the processed information to the surface. The processed information may be transmitted to the surface either by conventional wirelines or by mud pulsed telemetry. In mud pulsed telemetry, the processed information is pulsed back to the surface through the mud column using a valve which opens and closes to produce the pulses. See U.S. Pat. No. 5,586,084. The transmission rate for mud pulsed telemetry, however, is limited.
The electronic components in the bottom hole assembly are also limited in the temperature that they can withstand. Once the environment of the electronic components is subjected to high temperatures, such as 305.degree. F. or greater, for any extended period of time, some of the electronic components may stop functioning. Thus, electronic components, such as semiconductor chips, must be carefully produced and selected to ensure that they can withstand the anticipated heat, shock, and vibration of the bottom hole assembly. Since the life of the electronic components is a function of temperature over time, the higher the downhole temperature, the shorter the life of the electronic components. Thus, not only are the electronic components expensive, but the complexity of the equipment for processing the data downhole causes the bottom hole assemblies to be very expensive particularly for logging while drilling. Such electronic components also reduces the reliability of the bottom hole assembly.
In drilling new boreholes from existing wells to produce bypassed zones, it is often necessary to cut an aperture or window in the existing casing followed by a drilling string passing through the window to drill a deviated borehole into the bypassed zone. Prior art tools used in cutting the window in the existing casing produce a window of erratic geometry and often with an irregular shape. Also, the cutting tool tends to produce a jagged edge around the periphery of the window. Oftentimes successive trips are required into the borehole to clean up the window before the new deviated wellbore may be drilled. The irregular shape and jagged edge can cause problems in drilling the new borehole and completing the well. Since the specific location and geometry of the window is unknown, it is also difficult to establish a seal between the casing in the existing borehole and the new casing in the new borehole.
The prior art procedures for sealing the cased borehole with the new casing include filling the gaps between the irregularly shaped window and new casing with cement during the cementing operation. Special cement that is very plastic is often required for flowing into these gaps. Oftentimes the end of the casing must be milled clean. Also often the gaps remain around the window even after the cementing operation such that the cement still may not provide an adequate seal.
The present invention overcomes the deficiencies of the prior art.